1. Field of the Invention
The invention relates to a layer system having at least one metal oxide layer, the metal oxide comprising a titanium aluminum oxide. The layer system has a high structural and thermal stability, in particular at operating temperatures of over 600° C., and is especially suitable for optical coatings, without being restricted to this particular application.
2. Background of the Invention
Optical layer systems, in particular alternating layer systems, which are composed of alternating thin films with high and low refractive indices on top of one another, have been known for years for a wide range of applications. They act as a light interference film, the optical properties of which are determined by the choice of material for the layer with a high and/or low refractive index and therefore the choice of the corresponding refractive index, by the arrangement of the individual layers and by the choice of the individual layer thicknesses. The selection is substantially based on known optical design rules and design aids in accordance with the criteria of the desired optical properties and the processing properties.
Examples of suitable starting materials for the production of coatings of this type for, for example, reflectors, mirrors, filters, lamps, IRC torches/lamps, etc. include, inter alia, TiO2 and SiO2.
Thin films of these materials are usually applied by CVD processes (CVD=Chemical Vapor Deposition), PVD processes (PVD=Physical Vapor Deposition) or sol-gel processes and are distinguished by the fact that they are hard and chemically stable and have a high refractive index difference.
With regard to stability with respect to high temperatures, few problems are expected with silicon oxides. However, if titanium oxide is used as layer material with a high refractive index, problems arise at high operating temperatures (>600° C.), as may be reached, for example, for glass-ceramic reflectors or IRC lamps. The titanium oxide changes at these high temperatures. This is attributable in particular, as described extensively by H. Sankur and W. Gunning in J. Appl. Phys. 66 (1989), to the phase transformation of the TiO2 applied in the amorphous state. At temperatures of over 350° C., TiO2 crystallizes to form the anatase phase, then the rutile phase at temperatures around 600° C. This produces turbidity in the coating, and its optical properties, which have been optimized by means of a specific optical design, deteriorate.
DE 3227069 A1 discloses an optical coating which is suitable for high temperatures and comprises an alternating layer system made up of SiO2 as layer material with a low refractive index and Ta2O5 as layer material with a high refractive index. This coating is able to withstand high temperatures and is aimed at improving the working efficiency of the coated object, in particular of halogen lamps.
An improvement to the working efficiency and a higher thermal stability compared to the use of TiO2 as layer with a high refractive index can certainly be achieved, but using Ta2O5 in the high-temperature range also causes considerable turbidity in the layers, and consequently these coatings are unsuitable for many application areas.